For many applications, polyethylene with enhanced toughness, strength and environmental stress cracking resistance (ESCR) is important. These enhanced properties are more readily attainable with high molecular weight polyethylene. However, as the molecular weight of the polymer increases, the processability of the resin decreases. By providing a polymer with a broad or bimodal molecular weight distribution, the desired properties that are characteristic of high molecular weight resin are retained while processability, particularly extrudibility, is improved.
There are several methods for the production of bimodal or broad molecular weight distribution resins: melt blending, reactor in series configuration, or single reactor with dual site catalysts. Use of a dual site catalyst for the production of a bimodal resin in a single reactor is also known.
Chromium catalysts for use in polyolefin production tend to broaden the molecular weight distribution and can in some cases produce bimodal molecular weight distribution, but usually the low molecular part of these resins contains a substantial amount of the co-monomer. While a broadened molecular weight distribution provides acceptable processing properties, a bimodal molecular weight distribution can provide excellent properties. In some cases, it is even possible to regulate the amount of high and low molecular weight fraction and thereby regulate the mechanical properties.
Ziegler-Natta catalysts are known to be capable of producing bimodal polyethylene using two reactors in series. Typically, in a first reactor, a low molecular weight homopolymer is formed by reaction between hydrogen and ethylene in the presence of the Ziegler-Natta catalyst. It is essential that excess hydrogen be used in this process and, as a result, it is necessary to remove all the hydrogen from the first reactor before the products are passed to the second reactor. In the second reactor, a copolymer of ethylene and hexene is made so as to produce a high molecular weight polyethylene. The reverse configuration can also be used.
Metallocene catalysts are also known in the production of polyolefins. For example, EP-A-0619325 describes a process for preparing polyolefins such as polyethylene having a multimodal or at least bimodal molecular weight distribution. In this process, a catalyst system that includes at least two metallocenes is employed. The metallocenes used are, for example, a bis (cyclopentadienyl) zirconium dichloride and an ethylene bis (indenyl) zirconium dichloride. By using the two different metallocene catalysts in the same reactor, a molecular weight distribution that is at least bimodal can be obtained. Alternatively, a single metallocene catalyst component can be used in two serially connected loop reactors operated under different polymerization conditions. For example, the low molecular weight fraction is prepared in the first loop reactor in the presence of hydrogen and the high molecular weight fraction is prepared in the second loop reactor in the presence of a co-monomer, or vice versa.
Several resins or resin blends have been used in blow molding applications, but none of them provide an excellent compromise of contact transparency, gloss, impact resistance, good processing, environmental stress crack resistance (ESCR) and rigidity. For example, most high density polyethylene (HDPE) lacks contact clarity and gloss. Metallocene-prepared high density polyethylene (mHDPE) having densities larger than 0.940 g/cm3 lack ESCR, whereas metallocene-prepared medium density polyethylene (mMDPE) having densities ranging from 0.930 to 0.940 g/cm3 lack rigidity.
It is also known that co-extrusion is detrimental to transparency, that resins like PET cannot provide a broad range of container shapes and that clarified polypropylene has low impact resistance and poor processing capabilities.
There is thus a need to provide resins having an improved compromise between optical and mechanical properties as well as good processing capabilities.